The invention concerns a process for photolithographic generation of structures in the range below 200 nm.
In semiconductor technology and in microelectronics, the dimensions of structures are becoming smaller and smaller. In memory production today, structures with a width of less than 400 nm are produced using optical lithography in combination with the masking technique. A limit can be expected at approx. 150 nm in optical lithography because of diffraction effects. However, structures with even smaller dimensions are required for new applications such as single-electron transistors or molecular electronic components. In the case of very high-frequency circuits this is also true in conventional electronics.
One method offering possibilities for generating such small structures is X-ray lithography, which makes it possible to image dimensions of less than 100 nm--because of the shorter wavelength. However, this leads to problems with the required masks and in positioning. This is not the case with electron and ion beam lithography. Since these are direct writing methods, they do not require any masks. In electron and ion beam lithography, structures as small as 10 nm can be generated with high-energy particles. However, this requires expensive vacuum systems and beam guidance systems. In addition, problems can occur with sensitive components due to radiation damage in the substrate, because the high-energy particles can penetrate through the resist layers required for etching processes.
A novel possibility for high-resolution structuring is presented by scanning near-field techniques, in particular scanning tunneling microscopy (STM) and scanning force microscopy (SFM). With regard to writing speed, these techniques are in principle slower than electron beam writing methods but it is possible to work in parallel with a series of near-field probes. In addition, the writing speed and thus the time should not be crucial for initial developments because the method yields the great advantage that expensive vacuum systems can be eliminated.
With scanning near-field techniques, a fine pointed probe is moved over the surface of the specimen at a constant distance, and topographic differences can be compensated in this way. Interactions between the specimen surface and the tip of the probe serve to regulate the distance. When these techniques, which are usually employed only for scanning surface topographies, are to be used as structuring methods, then a current flow is generated, using an externally applied voltage, from the tip of the probe into the specimen or vice versa, depending on the polarity of the voltage, causing a chemical or physical change in the specimen surface. Since the distance between the tip of the probe and the surface of the specimen is extremely small, collisions between the electrons emitted and the molecules of air can be disregarded, and therefore electron exposure of the specimen surface is possible with scanning near-field techniques not only in high vacuum but also at normal pressure. This is an important advantage in comparison with the actual electron beam writing, where high vacuum is required--to prevent collisions between electrons and residual gas molecules in the acceleration zone (1-50 kV)--which entails a considerable expenditure.
With scanning near-field techniques, the electrons emitted cannot be accelerated to a high kinetic energy. In other words, if the voltage applied to the probe is too high (approx. &gt;80 V), uncontrolled damage may occur to the specimen and probe. Thus only electrons with an energy of up to approx. 80 eV may be used for electron bombardment with scanning near-field probes. This energy level is sufficient to initiate chemical changes in conventional electron beam-sensitive resist materials, but it is not great enough for the electrons to penetrate through dielectric resist layers more than a few 10 nm thick. The advantage that radiation damage in the substrate is ruled out has thus so far been outweighed by the disadvantage that only extremely thin resist layers can be used.
In microelectronics, however, plasma processing methods such as reactive ion etching (RIE) are conventional and require structurable, etching-resistant masks in the form of resists with a layer thickness of &gt;100 nm, depending on the etching depth of the structures to be produced.
Therefore, when using scanning near-field techniques, it is necessary to either refrain from substrate etching by plasma etching processes, in which case, however, the aspect ratio of the structures produced (height/width) is limited to values of &lt;1--or the etching processes must be performed in such a way that the etching depth does not exceed the thickness of the resist (see P. Avouris (ed.) Atomic and Nanometer-Scale Modification of Materials. Fundamentals and Applications, Kluwer Academic Publishers, 1993, pages 139-148). Another possibility consists of using metal halides, in particular calcium fluoride (CaF.sub.2), which have the etching stability required for substrate etching processes even in a thin layer (see Journal of Vacuum Science & Technology B, vol. 5 (1987), pages 430-433). However, the lithographic properties of such inorganic materials are very poor, in particular the dose required for structuring is very high, amounting to 1 C/cm.sup.2 for a 20 nm thick CaF.sub.2 resist, for example. This in turn limits the writing speed and therefore the throughput.